The invention relates to a method of manufacturing an optical conductor. It relates more particularly to setting the coverings on optical fibers by exposing them to ultraviolet radiation.
An optical conductor for conveying data comprises a glass fiber having a diameter of about one hundred microns, typically of 125 .mu.m, and at least one protective covering generally made up of two insulating layers. The protective covering is itself covered with a coloring layer for fiber coding or recognition purposes. The outside diameter of such a conductor lies in the range 245 .mu.m to 250 .mu.m.
A plurality of optical conductors can be associated in parallel to form a ribbon. In which case, coating is provided to hold the conductors together.
The coverings and the coating are made of polymers that can be cured (i.e. cross-linked) by exposing them to ultraviolet radiation.
In practice, to make the covering of a conductor, the fibers are surrounded in a layer of liquid polymer, and that material is solidified by being exposed to ultraviolet radiation. The two protective layers can be manufactured either serially, i.e. the first layer is manufactured prior to the second layer, or else in parallel, with both layers being deposited simultaneously and being cured together. Currently, the coloring layer and the coating covering (for forming a ribbon) are manufactured serially, one after the other.
To achieve the cross-linking, photo-initiator elements are inserted into the material to be polymerized. By means of the photo-initiators, curing by exposure to ultraviolet radiation causes a chemical polymerization reaction to take place, thereby setting the material. For example, the material to be polymerized can be an acrylate.
Curing by exposure to ultraviolet radiation is generally performed by means of two arc lamps. A first lamp produces radiation, the most intense wavelengths of which lie approximately in the range 350 nm to 400 nm. The first lamp principally performs in-depth cross-linking. A second arc lamp emits ultraviolet radiation that is rich in shorter wavelengths, of about 300 nm. The second lamp principally performs surface cross-linking.
The arc lamps give off large quantities of heat. It is thus necessary to cool them. It is also necessary to cool the optical conductor being manufactured because, at the end of the first exposure to ultraviolet radiation, the temperature of the covering is high and lies in the range 100.degree. C. to 1000.degree. C., and cross-linking does not take place correctly at high temperatures. Unfortunately, the reduction in temperature that can be obtained is relatively small, in particular because optical conductors must be manufactured at high speed, and often at speeds exceeding 100 meters per minute (m/min). Under such conditions, the quantity of photo-initiator material that needs to be used is greater than the quantity that would be necessary if the temperature were close to ambient.